Data signals which are transmitted by cable from one data transmission unit to another suffer degradations in the form of a general attenuation of the signal voltage level and a distortion of the pulse shape, both of which are functions of the cable parameters and the data rate. These effects are generally compensated for at the receiving unit by a cable equalizer.
Voltage level can be restored by means of an automatic gain control (AGC) circuit, which typically makes use of a single junction field-effect transistor (JFET) for gain control. At a data rate of 4.8 kb/s (kilobits per second), cables with only 0-14 db (decibel) insertion loss, generally known as "short" cables, require only gain for restoration, since the shape distortion is not severe enough to be significant. Cables with a 15-31 db insertion loss, generally known as "long" cables, require both gain and shape restoration. The shape restoration involves a gain component relative to the spreading of the received pulse shape and typically requires the use of another AGC circuit with a JFET. It might appear to be feasible to have one kind of equalizer providing only gain for short cables and another kind of equalizer with both gain and shape restoration for long cables. However, as PG,3 a practical matter, even for applications in which an equalizer is permanently connected to a single cable, it is advantageous to minimize the variety of equipment needed by the installer by supplying a single equalizer which can be manually switched as appropriate for the cable length or which is designed to automatically adjust for it. Additionally, in many other applications the equalizer is part of a data unit which may at a given time be receiving signals from a plurality of different cables and therefore must be capable of rapidly automatically adjusting accordingly within a range of about 31 db (decibels).
The useful range of a JFET device of the type generally used as the variable resistance control element for the AGC circuits in equalizers is about 25 db. In order to obtain the desired 31 db gain restoration range, there may be provided a pair of AGC circuits in a closely interactive configuration. Such arrangements are described, for example, in U.S. Pat. No. 3,824,501 issued July 16, 1974 to C. A. Harris and in U.S. Pat. No. 3,652,952, issued Mar. 28, 1972 to W. Chen, both of which are assigned to the same assignee as is the present invention. One drawback of such a closely interactive arrangement, however, is that the operating characteristics of the two JFET devices of the AGC circuits must be very closely matched in order to obtain the desired total gain range. This requirement reduces the reliability of the equalizer and significantly adds to its cost.